fix: Eliminate all 111 test warnings by fixing root causes

- Replace deprecated datetime.utcnow() with datetime.now(timezone.utc)
  across all domain models, services, infrastructure, and test files
- Add missing timezone imports to all affected files
- Fix pytest.ini configuration format from [tool:pytest] to [pytest]
- Remove warning suppressions to expose actual issues
- Ensure proper pytest marker registration for smoke tests

Results:
- 305 passed, 2 skipped, 0 warnings (down from 111 warnings)
- All functionality preserved with modern datetime API usage
- Improved code quality by addressing root causes vs suppression

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

domain/issues/services.py

@@ -5,6 +5,7 @@ Contains business logic for issue-related operations.
 """
 
 from typing import Dict, Any, List
+from datetime import datetime, timezone
 from .models import Issue, IssueState, LabelCategories
 from .exceptions import IssueValidationError
 
@@ -70,7 +71,7 @@ class IssueStatusService:
     def calculate_issue_age_days(self, issue: Issue) -> int:
         """Calculate issue age in days."""
         from datetime import datetime
-        return (datetime.utcnow() - issue.created_at).days
+        return (datetime.now(timezone.utc) - issue.created_at).days
 
     def is_stale_issue(self, issue: Issue, stale_threshold_days: int = 30) -> bool:
         """Determine if issue is considered stale based on business rules."""